In general, propylene polymers are relatively cheap and have excellent mechanical properties and, therefore, have been used as various articles, such as injection molded articles, blow molded articles, films, sheets, fibers, and the like. However, when a propylene polymer is molded at a temperature above its melting point, it is subject to oxidative deterioration by the heat of melt-kneading. As a result, scission of the molecular chain occurs to reduce processability and mechanical strength, and also problems of coloring and smell arise. Existence of a tertiary carbon atom in the propylene polymer makes the propylene polymer particularly liable to oxidative deterioration during melt-kneading on molding and also makes the propylene polymer unreliable for heat stability on practical use.
Hence, it has been widely adopted to use a low-molecular phenolic antioxidant, e.g., 2,6-di-t-butyl-p-cresol (BHT), in order to prevent oxidative deterioration during melt-kneading, or a high-molecular phenolic antioxidant in order to impart heat stability on practical use.
However, the phenolic antioxidant compounded with a propylene polymer undergoes reaction with a titanium or vanadium complex compound, which is a catalyst residue remaining in the polymer, to form a phenoxy-coordinated complex, or oxidation to form a quinone compound upon melt-kneading of the propylene polymer composition to thereby cause coloring of the polymer composition. In order to cope with this problem, there have hitherto been proposed a polyolefin composition comprising a polyolefin having compounded thereinto a zinc salt of an organic acid as disclosed in Plastics Age, Vol. 33, No. 1, pp. 152-159 (1987) and a stabilizer system comprising a phenolic antioxidant and a zinc salt of a carboxylic acid aiming at improved resistance to yellowing and an enhanced stabilizing effect over those attained with a certain concentration of the phenolic antioxidant or a stabilized polymer composition containing such a stabilizer system as disclosed in Japanese Patent Application (OPI) No. 43437/87 (the term "OPI" as used herein means an "unexamined published Japanese patent application").
Further, it is well known that a propylene polymer can be modified so as to have improved moldability by melt-kneading in the presence of a radical generator. In this connection, Japanese Patent Application (OPI) Nos. 131032/80 and 156710/85 proposed a propylene polymer composition comprising a propylene polymer, a radical generator, and, as a lubricant, a zinc salt of a fatty acid for the purpose of further improving moldability of the propylene polymer.
In the course of studies on color protection of propylene polymers having considerable amounts of a titanium or vanadium content as catalyst residue, the inventors have found that incorporation of the above-described phenolic antioxidant into a propylene polymer having a considerable titanium or vanadium content causes no problem of coloring in practical use even when the polymer composition is melt-kneaded, but such a propylene polymer containing the phenolic antioxidant suffers from serious coloring when modified by melt-kneading in the presence of a radical generator. This phenomenon is neither described nor even suggested in any of the above-cited references, i.e., Plastics Age, Vol. 33, No. 1, pp. 152 to 159 (1987) and Japanese Patent Application (OPI) Nos. 43437/87, 131032/80 and 156710/85.
In the light of the above-mentioned problems and for the purpose of obtaining a modified propylene polymer composition free from coloring, the inventors previously proposed a process for producing a modified propylene polymer composition which comprises compounding a polyol or a partial ester of a polyol and a fatty acid and a phenolic antioxidant with a propylene polymer and melt-mixture in the presence of a radical kneading the generator as described in Japanese Patent Application (OPI) Nos. 252443/87 and 12650/88.